Why do I have stretch marks?

None of my family members have stretch marks, mom never had them nor my grandmothers. I’m not even overweight if anything I’m actually a little on the thin side. (120 lbs and 5’4) I am curvy tho. But still. It’s not fair I’ve had them since I was 13. There deep purple on my hips. And I had some on my boobs too. And the worst thing is they haven’t gone away and I’m 20.